Add ability to copy assistant code block to clipboard or insert into editor, without manual selection (#17853)

Some notes:

- You can put the cursor on the start or end line with triple backticks,
it doesn't actually have to be inside the block.
- Placing the cursor outside of a code block does nothing.
- Code blocks are determined by counting triple backticks pairs from
either start or end of buffer, and nothing else.
- If you manually select something, the selection takes precedence over
any code blocks.

Release Notes:

- Added the ability to copy surrounding code blocks in the assistant
panel into the clipboard, or inserting them directly into the editor,
without manually selecting. Place cursor anywhere in a code block
(marked by triple backticks) and use the `assistant::CopyCode` action
(`cmd-k c` / `ctrl-k c`) to copy to the clipboard, or the
`assistant::InsertIntoEditor` action (`cmd-<` / `ctrl-<`) to insert into
editor.

---------

Co-authored-by: Thorsten Ball <mrnugget@gmail.com>
Co-authored-by: Bennet <bennet@zed.dev>

crates/assistant/src/assistant.rs

@@ -58,6 +58,7 @@ actions!(
     [
         Assist,
         Split,
+        CopyCode,
         CycleMessageRole,
         QuoteSelection,
         InsertIntoEditor,